1. Field of the Invention
The present invention relates generally to well production equipment, and in particular to polish rod jack systems for oil well pumping units that utilize a sucker rod string to operate a bottom hole pump.
2. Description of the Prior Art
For wells used to bring liquids to the surface from subterranean liquid-bearing reservoirs whose pressure is insufficient to produce the formation fluids by natural means, a pumping system must be used to extract the liquids from the liquid-bearing reservoirs. In the case of pumping oil from wells, the type of pumping system most commonly used comprises a pump unit disposed at the bottom of the well, a drive unit or pump jack disposed adjacent the top of the well, and a string of sucker rods connecting the polish rod that is reciprocated by the drive unit or pump jack to a piston in the bottom hole pump unit. Furthermore, the drive unit of choice utilizes an oscillating horse head on a structural member referred to as a walking beam. U.S. Pat. Nos. 3,376,826; 3,051,237 and 4,296,678 are all examples of the use of a walking beam drive for a sucker string actuated pump.
Normally, oil well spacings are from about one well for every 40 acres, all the way down to about five acres per well when thick zones and shallow production is involved. Even with five acre spacing, sufficient room is available to set up a walking beam rig, and a rural environment is involved where aesthetics are not a significant factor. However, when oil is to be extracted at an urban location or a golf course or other area where land is expensive and/or aesthetics is a significant consideration, as many as 35 wells may be clustered in the middle of a five acre site, the wells being drilled straight down for approximately 500 feet, then whipstocked (angled) to bottom out in the production zone from 3,000 to 10,000 feet deep where the wells have about a 40 acre spacing. In such a situation, on the surface, a typical arrangement might have three rows of 11 wells each, with five feet spacing between wells and rows, making use of walking beam type pumping units impractical, even apart from environmental and appearance considerations which would be associated with the use of such a cluster of walking beam units.
As a result, an alternative pumping system must be utilized at cluster well sites. If it is still desired to utilize a sucker string-operated bottom hole pump, then the most commonly utilized mechanical alternative to the walking beam system is the "Phoenix" hydraulic pumping system of White-Hopkins, Inc., Chester, Okla., and an illustration of an equivalent type of unit can be found in U.S. Pat. No. 4,637,459. Such hydraulic pumping systems utilize a vertically oriented piston and cylinder unit to hydaulically lift the polish rod and sucker rod string, instead of an osciallating walking beam. Piston and cylinder units also have the advantage over a walking beam type pump in that they require less power to operate and can be more closely spaced together; however, they possess significant shortcomings from both an aesthetic and practical standpoint also. The most notable of these shortcomings are associated with the fact that a hydaulic jack lift system will project vertically anywhere from ten to 40 feet above ground, depending upon the stroke requirement. That is, a cluster of such towers may be as unattractive as walking beam units. Moreover, because sucker rods have to be periodically replaced, such as due to breakage, and since it is necessary to pull the polish rod in order to make stroke adjustments, a problem exists when the rod pulling rig must be brought in. In particular, rod pulling rigs are truck mounted and are, typically, at least eight feet across and 30 feet long, so that as many as from eight to ten of the surrounding wells must be shut down and their jack units removed in order for the pulling rig to gain access to the well whose rods are to be pulled. This need to inactivate a number of otherwise fully operational wells dramatically increases the costs associated with using such hydraulic jack lift rod pumping systems.
In view of the above deficiencies in hydraulic jack rod pumping systems, most cluster well installations utilize bottom hole pumps which are hydraulically driven from a surface power unit instead of a mechanical or hydraulic sucker rod type drive. One commonly used surface powered hydraulic down-hole pumping unit is the Kobe production unit produced by Kobe, Inc., Huntington Park, California. An example of another such down-hole hydraulic pumping system can be found illustrated in U.S. Pat. No. 4,403,919.
However, hydraulically driven systems pose their own disadvantages. Firstly, to hydraulic drive a bottom hole pump that may be 10,000 feet below the surface of the earth requires the use of high pressure pumps (having pressure ratings as high as 50,000 psi), as well as high pressure tubing for delivering the hydraulic fluid from the surface power unit to the down-pump and another string of tubing to return the hydraulic fluid back to the surface power unit (a return string is sometimes eliminated and the hydraulic fluid mixed with and returned with the production oil via the oil delivery tubing, but such requires the additional provision of a surface separation unit for separating the hydraulic fluid and production oil). Since, in some instances, it is desired to lift fluids from two or more subterranean formations by means of separate flow tubing strings in a common bore hole, the presence of such additional hydraulic fluid lines in addition to the necessary production oil delivery tubing poses problems in terms of the available space within the well casing.
Besides physical considerations, the cost of the equipment used to hydraulically drive a bottom hole pump is approximately twice that of mechanically driven down-hole pumping arrangements. However, perhaps the most costly and disadvantageous factor in using a bottom hole pump taht is hydraulically driven from a surface unit is that such systems lead to as much as a 50% increase in the malfunction rate of the bottom hole pump due to particles (scale) in the hydraulic lines being carried by the hydraulic fluid being transported down-hole to the pump, where they entering into the pump and prevent it from properly operating to an extent requiring the pump to be pulled for servicing. In this regard, it is also very significant that pump pulling operations for hydraulically driven bottom hole pumps involve the well being shut down for about four days, in comparison to an approximate six hour shutdown time when a mechanically driven bottom hole pumps malfunctions and must be pulled for servicing.
In view of the foregoing, it can be seen that there is a real need for a means for driving a bottom hole pump via a sucker rod string and polish rod which can be fully received within a well casing, so that the disadvantages of the above-mentioned types of pumping systems can be avoided wihtout creating other and at least equally disadvantageous conditions.